Envelope-receptor interactions of neurotropic HIV

Human immunodeficiency virus (HIV) infects macrophages and microglia in the central nervous system (CNS) and causes neurocognitive impairment in 10-20% of patients with acquired immune deficiency syndrome (AIDS). Tissue macrophages and brain microglia express lower levels of the HIV entry receptor CD4 than CD4+ T lymphocytes in peripheral blood, suggesting that reduced dependence on receptor levels for entry may enhance HIV macrophage/microglia tropism. Mechanisms by which HIV acquires an enhanced capacity to enter cells when receptor levels are low are unclear. In this dissertation, we sought to identify molecular determinants in primary, neurotropic HIV that enhance envelope glycoprotein (Env)-receptor interactions and virus entry in macrophages and microglia to better understand mechanisms of HIV neurotropism and pathogenesis. We identified 5 HIV Env variants that appeared at a high frequency in brain Envs. HIV Env variant N283 in the CD4 binding site of gp120 increases Env affinity for CD4 by decreasing the dissociation rate from CD4, enhancing HIV replication and cytopathic effects in macrophages and microglia. N283 is associated with brain infection and dementia in vivo , representing the first example of an HIV variant associated with a specific AIDS-related complication. An amino acid variant in the V4 region of gp120, D386, eliminates a glycosylation site and increases exposure of the b12 epitope, a conserved neutralizing epitope that overlaps the CD4 binding site, enhancing viral replication in macrophages and providing evidence that determinants of macrophage and microglia tropism are overlapping but distinct. HIV Env variants P308, T373, and S636 in the V3 and C3 regions of gp120 and HR2 region of gp41 do not contribute to reduced CD4 or CCR5 dependence, but identification of these variants will facilitate future studies on the contribution of enhanced Env-CCR5 interactions and increased fusogenicity to macrophage/microglia tropism. Together, these findings suggest that a key mechanism by which HIV acquires enhanced tropism for macrophages and microglia is enhanced Env-CD4 interactions via increased Env-CD4 affinity or exposure of the CD4 binding site. These studies provide a better understanding of Env-receptor interactions that contribute to HIV tropism and pathogenesis and suggest that therapeutics targeting Env-CD4 interactions may be beneficial for preventing CNS infection and neurologic injury in HIV-infected patients.